This error message is printed when a nondeclared variable is used in the code
running under the strict pragma. For example,
consider the short script below, which contains a use
strict; pragma and then shamelessly violates it:

Moreover, in certain situations (e.g., when SIG{_ _DIE_
_} is set to Carp::confess( )) the
entire script is printed to the error_log file
as code that the server has tried to evaluate, so if this script is
run repeatedly, the error_log file will grow
very fast and you may run out of disk space.

This problem can easily be avoided by always declaring variables
before using them. Here is the fixed version of our example: